1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS image sensor and a method for fabricating the same, which improves the sensor's image characteristics.
2. Discussion of the Related Art
Image sensors are semiconductor devices for converting an optical image into an electrical signal and include charge-coupled devices and complementary metal-oxide-semiconductor (CMOS) image sensors. A general charge-coupled device includes an array of photodiodes converting light signals into electrical signals. Disadvantages of a charge-coupled device include a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photo process. In a charge-coupled device, integration of complementary circuitry such as a control circuit, a signal processor, and an analog-to-digital converter into a single-chip device is difficult. Thus, development of compact-sized or thin products using such image sensors is hindered. Examples of compact-sized or thin products include digital still cameras and digital video cameras.
CMOS image sensors, on the other hand, adopt CMOS technology using a control circuit and a signal processing circuit as a peripheral circuit. CMOS image sensors also adopt switching technology which allows outputs to be sequentially detected using MOS transistors corresponding to a number of arrayed pixels. This allows an image to be detected. Accordingly, a CMOS image sensor uses CMOS fabrication technology, i.e., a simple fabrication method using fewer photolithography steps, thereby enabling an advantageous device exhibiting low power consumption.
In the aforementioned CMOS image sensor of the related art, the photodiode is the active device for forming an optical image based on incident light signals. The optical image is formed by generating electrical signals according to the intensity and wavelength or color of incident light. In such a CMOS image sensor, each photodiode senses incident light and the corresponding CMOS logic circuit converts the sensed light into an electrical signal according to input wavelength. The photosensitivity of the photodiode increases as more light is able to reach the photodiode. In this instance, enhanced photosensitivity results from an increase in the levels of sensed light and corresponds to the light-receiving capability of the active device. One way of enhancing the photosensitivity of a CMOS image sensor is to improve its “fill factor,” i.e., the degree of surface area covered by the photodiodes versus the entire surface area of the image sensor. The fill factor is improved by increasing the area responsive to incident light, i.e., the photo-sensing portion. However, there is a limit to increasing the photo-sensing portion due to the required presence of the logic circuit portion.
Therefore, a device of a material exhibiting excellent light transmittance, such as a convex microlens having a predetermined curvature for refracting incident light, may be provided to redirect any light that may be incident on the image sensor outside the immediate area of the photodiodes. The device may also be provided to concentrate or focus the incident light on one or more of the photodiodes themselves. That is, the incident light, striking the surface of the convex structure of the microlens while in parallel to the optical axis of the microlens, is refracted by the microlens according to the curvature of the convex structure. The incident light is thereby focused at a predetermined point along the optical axis. Accordingly, in a color image sensor, such a microlens may be provided over a color filter layer including red (R), blue (B), and green (G) filter elements for passing the light of each color or wavelength to be disposed over a photodiode area.
Meanwhile, CMOS image sensors are classified according to a number of transistors. For example, a 3T-type CMOS image sensor consists of one photodiode and three transistors and a 4T-type CMOS image sensor consists of one photodiode and four transistors. An equivalent circuit and layout of a unit pixel of a 3T-type CMOS image sensor of the related art are shown in FIG. 1 and FIG. 2, respectively.
Referring to FIG. 1, a CMOS image sensor of the related art comprises one photodiode PD and three NMOS transistors including a reset transistor Rx, a drive transistor Dx, and a select transistor Sx. The cathode of the photodiode PD is commonly connected to the drain of the reset transistor Rx and the gate of the drive transistor Dx, whose drain is connected to the source of the select transistor Sx, whose drain is in turn connected to a read circuit. With the anode of the photodiode PD grounded, a reference voltage VR is applied via a power line to the source of each of the reset and drive transistors Rx and Dx. A reset signal RST is applied via a reset line to the gate of the reset transistor Rx and a select signal SLCT is applied via a column select line to the gate of the select transistor Sx.
Referring to FIG. 2, an active area 100 is defined for each unit pixel of the CMOS image sensor of FIG. 1. The active area 100 includes a photodiode area 20 comprising the bulk of the active area, which is overlapped by gate electrode areas 120, 130, and 140 of the three NMOS transistors, respectively. The source/drain region of each transistor is formed by an ion-implantation process with respect to the active area 100. Power (Vdd) is applied to the source/drain regions of the reset transistor Rx and drive transistor Dx. The source/drain region of the select transistor Sx is connected to the read circuit. Each of the gate electrodes 120, 130, and 140 is connected to external circuitry (not shown) via a corresponding signal line having a pad provided at one end.
As shown in FIG. 3, a CMOS image sensor of the related art includes a plurality of photodiodes 11 formed in a surface of a semiconductor substrate 10, an insulating interlayer 12 formed on the entire surface of the semiconductor substrate 10 including the photodiodes 11, a first planarization layer (not shown) formed on the insulating interlayer 12, a color filter layer 14 formed on the first planarization layer, a second planarization layer 15 formed on the entire surface of the semiconductor substrate 10 including the color filter layer 14, and a plurality of microlenses 16 provided on the second planarization layer 15 corresponding to the respective photodiodes 11, each microlens being formed as convex structure and having a predetermined curvature. The microlenses 16 focus incident light onto the corresponding photodiodes 11 through the color filter layer 14. The color filter layer 14 is comprised of red (R), green (G), and blue (B) color filter patterns for respectively filtering light according to wavelength. Each of the photodiodes 11 is disposed below one of the color filter patterns and generates an electrical charge according to the amount of incident light that reaches the photodiode.
The curvature and height of the microlens, which is a critical component of an image sensor, are determined in due consideration of the desired focus of incident light. Generally, the microlens is made of polymer-based resin enabling a completed microlens to be formed using only a photolithography patterning process of exposure and development followed by a reflowing process. The pattern profile or shape of the microlens tends to vary according to exposure conditions, for example, the conditions of a thin film on the semiconductor substrate, which are rather unstable. This results in degraded focusing characteristics. The microlens is nevertheless formed to have the optimal size, thickness, and curvature radius, which are determined with due regard to such parameters as shape, size, and positioning of a unit pixel, photodiode thickness, and physical characteristics (e.g., dimensions) of a light-shielding layer.
Meanwhile, the respective color filter patterns have different heights since each is formed by its own individual photolithography processing. Therefore, the planarization layer 15 provides a level surface, above the upper surfaces of the color filter patterns, for receiving the microlens array. Accordingly, as shown in FIG. 3, the traveling distance a of light, which passes through the microlens 16 to be incident on the photodiode 11, is inherently increased due to the thickness of the planarization layer 15. Since the light-receiving efficiency of the CMOS image sensor of the related art depends on the amount of light reaching the photodiode, light sensitivity is lowered due to the greater traveling distance, thereby deteriorating the capacity of the CMOS image sensor.